Germination Structure of Seeds Quote:
Thomas Fuller's Gnomologia, 1732: "The greatest Oaks have been little Acorns."
Seeds are found in a staggering array of shapes and sizes, but the process by which seeds germinate is similar in all species.
Abies koreana Acer griseum Wisteria floribunda
'Korean Fir' by Roger Culos. CC BY-SA. 'Paperbark Maple' by Roger Culos. CC 'Japanese Wisteria' by Roger BY-SA. Culos. CC BY-SA.
Alsomitra macrocarpa Macrozamia communis Strelitzia reginae
'Alsomitra macrocarpa seed' by Scott 'BurrawangSeeds' by AYArktos. CC BY- 'Paradiesvogelblumensamen' by Zona. CC BY. SA. Sebastian Stabinger. CC BY-SA.
Taraxicum officinale Stephanotis floribunda Phleum pratense
'Achane of Taraxacum sect. 'Stephanotis seed' by L. Marie"/Lenore 'Timoteegras vruchten Phleum Ruderalia' by Didier Edman, Sunnyvale, CA. CC BY. pratense' by Rasbak. CC BY-SA. Descouens. CC BY-SA.
Dicotyledon seeds
testa epicotyl
plumule hypocotyl
'Aesculus hippocastanum seed section' by Boronian. CC BY.
plumule epicotyl
hypocotyl testa
hilum
radicle
cotyledon micropyle
Monocotyledon seeds
endosperm
epicotyl testa
hypocotyl
cotyledon radicle
Parts of a seed
Testa The seed coat. A protective layer which is tough and hard and it protects the seed from attack by insects, fungi and bacteria.
Cotyledon
Dicotyledons have 2 cotyledons Monocotyledons have 1 cotyledon
A cotyledon is an embryonic leaf. It is the first leaf to appear when a seedling grows. They often contain reserves of food which the developing seedling can use to grow.
Epicotyl The section of stem between the cotyledon(s) and the plumule. In a seedling it is the section of stem between the cotyledons and the first true leaves.
Hypocotyl The section of stem below the cotyledon(s) and above the radicle, or root in a seedling.
Radicle The first part of the embryo to emerge when a seed germinates, it becomes the root.
Plumule The embryonic growing shoot and first leaves of a plant. This is the part of the plant that grows above the cotyledon(s) when the seedling develops.
Hilum The scar where the seed was attached to the parent plant.
Endosperm The food store made up of protein and starch which feeds the developing seedling before it is able to uptake water and nutrients through its own roots.
Micropyle This is a tiny pore in the testa located just opposite the radicle which lets water into the seed.
Seed Structure Activity
Fill in the names of the seed parts on the diagram below
hilum epicotyl radicle endosperm testa plumule cotyledon hypocotyl micropyle
Seed Dormancy
Normally when environmental conditions are favourable a seed will begin to germinate, but in some cases a seed can remain dormant even though the conditions are perfect for germination.
Why?
To delay germination
Why?
To cause germination to be staggered which allows seeds to travel different distances before they germinate. This ensures a wide geographical spread so that seeds germinate at different times, protecting some of the seedlings against short periods of inclement weather or passing herbivores.
Video link: Some seeds can germinate https://www.youtube.com/watch?v=JRc7FAHiNOo after 2000 years of dormancy!
Seed dormancy can be primary or secondary
Primary dormancy Secondary dormancy
Seeds are released from Seeds are not dormant the plant in a dormant when they are released state from the plant but become dormant if environmental conditions are unfavourable
Two types of seed dormancy
Coat Imposed Dormancy Embryo Dormancy
o Prevention of water uptake o Embryo inhibits germination o Mechanical constraint through the presence of o Interference with gas exchange growth inhibitors and the
o Retention of inhibitors lack of growth promoters. o Inhibitor production
Seed Dormancy Quiz
Video link: https://www.youtube.com/watch?v=scU9NsEtHn8
Watch the video and then see if you can answer these questions.
1. What is seed dormancy? a. Seeds from unfertilised flowers that cannot germinate b. Seeds that do not germinate even when environmental conditions are correct c. Seeds that have not formed properly and so cannot germinate
2. What advantage does seed dormancy give to plants? a. It protects them from attack by fungi and bacteria b. It makes them better for human consumption c. It staggers timing of germination to allow for dispersal
3. What is primary dormancy? a. Seeds are in a dormant state when they are released from the plant b. Seeds become dormant when the environmental conditions are unfavourable c. Seeds are in the first of a series of dormant phases
4. What is secondary dormancy? a. Seeds are in a dormant state when they are released from the plant b. Seeds become dormant when the environmental conditions are unfavourable c. Seeds are in the first of a series of dormant phases
5. What are the two types of seed dormancy? a. Coat imposed dormancy and embryo dormancy b. Coat imposed dormancy and root dormancy c. Root dormancy and embryo dormancy
Stimuli that break dormancy
The seeds from different species of plant are brought out of dormancy by a variety of stimuli. Most seeds respond to more than one stimuli, so mimicking one condition when breaking seed dormancy may not be enough.
After ripening Chilling Light Some seeds germinate once Some seeds need a cold Many seeds germinate when they have dried out to a certain treatment to germinate they are exposed to light. This which mimics the seeds going extent. For the majority of may be a brief exposure, through winter. Once the intermittent exposure, or seeds if they dry out so that cold spell is over they they contain less than 5% water germinate in the spring. The exposure in a specific pattern they can be damaged. process of chilling seeds is such as short or long day length. often referred to as Fire stratification. Water Some seeds require extreme Seeds need to absorb water high temperatures for the seed Oxygen (imbibition) to begin the process coat to be broken to allow Seeds need to absorb oxygen of germinating. germination to take place. to begin the process of germinating. Without oxygen Warmth Digestion the seed cannot carry out Seeds from different species Some seeds will not germinate respiration which provides require different minimum unless they have been eaten temperatures to germinate. and excreted. the energy for growth.
Imbibition
'The breaking of the seed coat' by Mr Revolution. CC BY.
When a seed comes into contact with water it absorbs it and swells up. The seed imbibes water. Imbibe means ‘to drink’. Do not get this process mixed up with osmosis, it is not the same thing!
This is possible because of the presence of colloidal particles in the testa which attract water and cause the seed to swell.
Watch this video, paying particular attention to the seeds imbibing before they start to germinate.
Video link: https://www.youtube.com/watch?v=pB4ASdELBbQ
Do not get the process of Osmosis is the movement of imbibition mixed up with the water across a semi-permeable process of osmosis, it is not the membrane from an area of low same thing! solute concentration to an area
of high solute concentration
Factors limiting imbibition
Temperature: The rate of imbibition increases with temperature increase. Solute concentration: The purer the water is, the higher the rate of imbibition. If the water is high in solutes (dissolved compounds) then the rate of imbibition will be low.
Why is imbibition so important?
It plays a key role in the initial stage of water absorption by roots when seedlings are young.
It initiates seed germination.
It plays a role in adhering water to xylem tissues.
It helps fruits to retain water.
StorageIt plays Tissues a major role in young and actively growing tissues.
The endosperm in the seed is a store for energy that allows the seed to grow and the seedling to develop to the point that it can feed itself from its roots.
What is in the endosperm? Endo = ‘within’ Starch Sperm = ‘seed’ Protein Sometimes Oils and Fats Endosperm = ‘within seed’
During germination the embryo submits signals to the endosperm which causes it to start breaking down and releasing nutrients. The endosperm also breaks down structurally which allows the cotyledons to emerge from the seed.
The endosperm is able to recognise environmental signals and it can give signals to the embryo to regulate its growth according to the environment around the seed.
Some tiny, dust-like, seeds have no endosperm
Substrate breakdown in the endosperm:
Enzymes in the seed which become active when the seed imbibes water break down the large storage molecules of starch in the endosperm and cotyledons. The large molecules are broken into smaller molecules of glucose which can be used for respiration to produce energy for growth. Specific enzymes which loosen the cell wall are also produced during this phase of germination to prepare for the radicle to emerge from the seed.
Advanced reading activity: http://aggie- horticulture.tamu.edu/faculty/davies/pdf%20stuff/ph%20final%20galley/Chap%207- %20M07_DAVI4493_00_SE_C07.pdf
Seed to Seedling Video link: https://www.youtube.com/watch?v=Sxjv18W9cDQ
Radicle Emergence
'Sprouted chickpea stages' by Edukeralam. CC BY-SA.
When you have sown seeds the first noticeable indication of germination is the appearance of the radicle which protrudes from the seed and starts to grow down. To start with the radicle growth is caused by cell elongation, but as the radicle continues to grow the cells start to divide at the growing tip.
Shoot Emergence
'Seed germination' by U.S. Department of Agriculture. CC BY.
Once the radicle has emerged and started to grow down into the soil the shoot begins to emerge. The emerging seedling uses food reserves from the endosperm and cotyledons to grow the hypocotyl region which lengthens the seedling stem and pushes it out of the seed and towards the light. The cotyledons (single cotyledon if it is a monocotyledonous plant) then unfold and you can see your seedling. The epicotyl then starts to develop and you see the first true leaves begin to emerge and the seedling can then start feeding itself through uptake of nutrients and water from the soil through its roots and photosynthesis in its leaves.
Heterotrophic growth Autotrophic growth
This is when the germinating This is when the seedling is old seed and developing seed is enough and can feed itself from feeding from organic material i.e. water and nutrients taken up
using the stored food in the through its roots and endosperm and cotyledons photosynthesis.
Environmental Factors
Temperature Water Three temperature points: Seeds need moist conditions to Minimum · Optimum · Maximum germinate, but not too wet, and Germination occurs most rapidly at the seedlings are very sensitive to drying out. optimum (perfect) temperature. Below But both seeds and seedlings can be the minimum, and above the maximum killed by waterlogging as if the temperature germination will not occur. germinating seed, or the root of the Each species of plant has a different young seedling, becomes completely minimum, optimum and maximum surrounded by water then they cannot temperature. uptake gases for respiration.
Gases Light Oxygen uptake is essential for Many plant species which produce small germination but becomes impossible for seeds need light to trigger germination. For some it is the correct wavelength of the seeds if the soil is too compacted or the light, and for some it is the correct becomes waterlogged. If a hard crust photoperiod (day length) that triggers forms on the soil surface then oxygen germination. Small seeds do not have big diffusion into the soil is limited, and this food reserves and so the epicotyl needs reduces germination rates. Some water to reach the soil surface quickly so that plant seeds can only germinate in water the first leaves can start to photosynthesis to feed the seedling. and are inhibited by exposure to air.
Images for temperature:
or and
Image for water:
and
Image for gases:
Image for light:
Seed Technology
A seed technology laboratory
Image by Orin hargraves. CC BY. Genetically modified seed which is resistant to herbicides Seed technology is the improvement of physical or genetic properties of seeds and the plants they grow into. It is particularly important in agriculture as disease resistance, drought resistance, herbicide resistance, physical strength and many other attributes are important properties to ensure reliable, high yielding crops.
Genetic modification of plants is a controversial, although very widely used, technique. One side of the debate states that it is an unnatural process and a threat to native ecosystems, the other side states that it is a necessary process to be able to feed the world’s rapidly increasing population.
Reading activity: http://www.senseaboutscience.org/data/files/resources/9/MSofGM2011.pdf
Reference: Fuller, T., 1732. Gnomologia: Adagies and Proverbs; Wise Sentences and Witty Sayings, Ancient and Modern, Foreign and British. B. Barker; and A. Bettesworth and C. Hitch.